Chemical vapor deposition ("CVD") has been used for a number of years to make diamond or diamond-like coatings (see, e.g., A. Feldman et al. "Chemical Vapor Deposited Diamond," pp. 413-436 in Ceramic Films and Coatings, ed. by J. B. Wachtman and R. A. Haber (Noyes, Park Ridge, N.J., 1993)). These coatings have found use in cutting tools for machining aluminum and other ductile nonferrous alloys, plastics, and abrasive composite materials (see, e.g., R. A. Hay, "The New Diamond Technology and its Application in Cutting Tools," pp. 305-327 in Ceramic Cutting Tools, ed. by E. D. Whitney (Noyes, Park Ridge, N.J., 1994)).
It is known that cobalt interferes with adhesion of WC--Co inserts and many techniques have been devised to try and improve the adhesion of diamond to cemented carbides (see U.S. Pat. No. 5,236,740 to Peters et al. (Aug. 17, 1993) which teaches a method for increasing the surface roughness to improve adhesion via chemical etching, U.S. Pat. No. 5,433,977 which teaches deposition of a refractory intermediate layer to enhance adhesion, and U.S. Pat. No. 5,370,944 which teaches introducing a surface-modified layer containing no binder phase).
Silicon nitride and SiAlON ceramics have coefficients of thermal expansion ranging from 2.8.times.10.sup.-6 /.degree.C. to 3.7.times.10-6/.degree.C., having a good match with diamond at .apprxeq.3.2.times.10.sup.-6 /.degree.C. Such tools have little residual stress in the coatings. The reported fracture toughness of a polycrystalline coarse-grained diamond CVD coating is 5.3.+-.11.3 MPa.sqroot.m at room temperature (see Drory, et al., "Fracture Toughness of Chemically Vapor-Deposited Diamond," J. Am. Ceram. Soc., 7412! 3148-50 (1991)). Typical silicon nitride and SiAlON substrates range in toughness between 4 and 6 MPa.sqroot.m. These materials have low apparent toughness compared to tools with residual compressive stresses.
Tungsten carbide 6 wt. % Co substrates have a coefficient of thermal expansion of .apprxeq.5.times.10.sup.-6 /.degree.C. and can therefore be used to put the diamond coating in compression. Unfortunately, the poor adhesion at the interface limits these coatings thicknesses, such that either it is impractical to deposit thick coatings due to slow deposition rates or the coatings spall off due to the high residual stresses at the weakly bonded interfaces. The toughness of WC--Co substrates are typically on the order of 8 MPa.sqroot.m.
Binderless WC has been used for machining titanium with some success (see U.S. Pat. No. 4,828,584 to Cutler (May 9, 1989)). Cutler teaches how to select a grain growth inhibitor to limit grain size. U.S. Pat. No. 5,089,447 to Timm et al. (Feb. 18, 1992)) used W.sub.2 C as an additive in order to control grain size and make high hardness, fine-grained materials. These materials have toughness values near 4 MPa.sqroot.m. U.S. Pat. No. 5,372,797 to Dunmead et al. (Dec. 13, 1994) relates to a method for making submicron WC such that very fine-grained materials can be made using this technology. While high hardness appears to be advantageous in water jet cutting, it is not requisite for a substrate since the diamond has higher hardness than the finest WC or WC-based composite. Accordingly, fine-grained WC materials are not currently believed to be used as substrate materials due to their low fracture toughness.
It has been demonstrated that compressive surface stresses can be used to improve performance when loading is concentrated at the surface which is under surface compression (see Chao et al., "Rolling-Contact Fatigue and Wear of CVD-SiC with Residual Surface Compression," J. Am. Ceram. Soc., 789! 2307-13 (1995)). Furthermore, recent calculations and experiments show that the apparent toughness of cracks extending through the layer under surface compression can result in higher values of apparent toughness than that of the monolithic materials (see Lakshminarayanan, et al. "Toughening of Layered Ceramic Composites with Residual Surface Compression," J. Am. Ceram. Soc., 791! 79-87 (1996)). Peak apparent toughness values on the order of 30 MPa.sqroot.m were obtained for Al.sub.2 O.sub.3 -15 vol. % ZrO.sub.2 composites with surface compression on the order of 400 MPa, whereas the intrinsic toughness of the material is on the order of 5-7 MPa.sqroot.m.
It would be an improvement in the art to have a ceramic or ceramic metal (i.e., cermet) component with a thicker and more adherent diamond coating so as to have improved wear life and increased manufacturing flexibility (e.g. a thicker diamond coating could be pre-ground and shaped more easily than a thinner diamond coating).